The present invention relates to a method of receiving wavelength multiplex signals, and particularly to a method of receiving wavelength multiplex signals transmitted from a plurality of signal sources.
In centralized remote monitoring for such industrial facilities as overhead power lines laid out over a long distance or wide area, various information is collected in many places and transmitted through optical fiber cables. In this case, the total quantity of information is assessed as a product of the number of kinds of information and the number of places. Although each piece of information is not complex, the total quantity of information is enormous, and their transmission and reception must be conducted efficiently. For example, when it is intended to assess the functional status of a power line at each moment of time in terms of a 10-step index, the status of each point is converted into one of ten light intensities, the optical signal is transmitted through an optical fiber cable, and it is received at a transformer station. If there are five types of fault indicated by five pieces of information collected at each of 100 power line towers, it is necessary to transmit 500 pieces of information separately, virtually simultaneously, and uninterruptedly.
One prevalent method of transmitting numerous signals efficiently through an optical fiber cable is the wavelength multiplexing optical signal transmission system, in which lights of different wavelengths are used to transmit different kinds of information simultaneously and the optical signals received by the receiving station are separated by means of wavelength filters. In the case of a single information source, signals are readily separated based on wavelength, whereas in the case of multiple information sources on power line towers along an optical fiber cable, identification of each information source is required in addition to the wavelength separation.
Methods of information source identification include: (1) Transmission of signals, with information source codes appended thereto, (2) Time-based signal transmission conducted at a predetermined time point for each information source, or sequential signal transmission based on a predetermined interval and sequence of transmission among information sources along the cable. Any of these methods allow for redundancy in sending the positional information.
Another method, which is intended to enhance the transmission efficiency or for the case where the quantity of information is too large to take control of location identification or appendage of location code, is that the receiving station sends a trigger signal accompanied by certain information and an information source responds to the trigger signal to send back information. This scheme is designed such that the receiving station sends a trigger signal, the signal reaches the nearest information source A, the source A transmits information signals in multiple wavelengths, and the receiving station completes the reception of the signals of source A before signals of a farther information source B come to the position of the source A following the arrival of the trigger signal at the source B. The receiving station receives information signals sequentially from the nearest source and then from the next nearest source and so on, and it can separate signals of each information source based on the wavelength.
Although the above-mentioned sequential reception of information signals from many places by using the trigger signal provides an efficient signal transmission path through the optimal setting of the timing condition for the trigger signal and information signals, it involves a problem of increased complexity of process for the received signals. The problem is mainly attributable to the fact that the velocity of light in the optical fiber cable is dependent on the wavelength of the light. Supposing the transmission of information .alpha..sub.i and .beta..sub.i from an information source with a distance of L.sub.i from the receiving station at a time point t.sub.o of the arrival of trigger signal by choosing wavelengths .lambda..sub..alpha. and .lambda..sub..beta. having velocities V.sub..alpha. and V.sub..beta., respectively, (where V.sub..alpha. &gt;V.sub..beta.) in the optical fiber cable, the receiving station will receive these information at time points EQU t.sub..alpha.i =t.sub.o +t.sub.c +L.sub.i /V.sub..alpha. and t.sub..beta.i =t.sub.o +L.sub.i /V.sub..beta. respectively.
The term t.sub.c is a time length expended by an information source before it transmits the information signal after receiving the trigger signal, and this time length may be zero in some cases.
FIG. 2 shows, on the horizontal time axis, information signals of the kinds .alpha. and .beta. received in correspondence to the distances L.sub.1, L.sub.2, . . , , L.sub.m at a virtually equal interval. The .alpha.-type information signals with a high velocity are received earlier than the .beta.-type information signals. On this account, when signals are sampled at a time point t.sub.p, the picked-up signals S.sub..alpha. (t.sub.p) and S.sub..beta. (t.sub.p) are information of kinds .alpha. and .beta. from the locations with distances L.sub.j and L.sub.i, i.e., these information signals are not of a same location. The greater the distance of an information source from the receiving station, the larger is the error in the information signal received from it. Accordingly, it is necessary to rearrange information signals sampled at a time point t.sub.p in consideration of error thereby to group information signals from each information source.
Accordingly, this method of efficient use of the optical fiber information transmission path necessitates the signal rearrangement process, and it suffers degraded efficiency as the whole reception system. Particularly, for the purpose of collecting information from many places distributed at a small interval along an optical fiber cable, it will become a very inefficient method of information collection to sample data at a constant time interval and thereafter rearrange numerous data so as to group data of individual information sources for the assessment of the functional status of each place. Moreover, in the example of FIG. 2, sampling at a time point t.sub.q provides data of kind .alpha. from one information source, but this time point is a gap section of reception of data of kind .beta. between two information sources, resulting in the deficiency of data reception or the erroneous identification of information source.